The present invention relates to a method for preparing improved copper aluminum borate catalyst More particularly, the invention involves producing copper aluminum borate by forming an aqueous composition comprising a volatile organic liquid having at least partial miscibility with water, a source of copper(II) ions, a source of alumina, and a source of boria at a pH in a range from about 4 to about 12, drying the composition to form a superficially dry solid, and calcining the dry solid at a sufficiently high temperature to form crystalline copper aluminum borate. Preferably, the process of producing copper aluminum borate comprises forming an aqueous composition comprising a source of copper(II) ions, a source of alumina, and a source of boria, admixing with the aqueous composition a volatile organic liquid containing a chemical base to form a homogeneous gel, drying the gel to form a superficially dry solid, and calcining the dry solid at a sufficiently high temperature to form crystalline copper aluminum borate. The present invention is also directed to a method for producing a copper aluminum borate precursor which when dried and/or calcined to a sufficiently high temperature forms crystalline copper aluminum borate.
Catalytically active copper aluminum borate which is at least partially reducible with hydrogen under Temperature Programmed Reduction (TPR) at a temperature of no more than 350.degree. C. and which has a surface area of at least 5 square meters per gram and a pore volume of at least 0.04 cc per gram is the subject of commonly assigned Satek U.S Pat. No. 4,590,324; of commonly assigned Kouba et al. U.S. Pat. No. 4,613,707; of commonly assigned Zletz et al. U.S Pat. No. 4,645,753; of commonly assigned Zletz U.S. Pat. No. 4,729,979; of commonly assigned De Simone et al. U.S Pat. No. 4,755,497; and of commonly assigned copending application of Zletz U.S. Ser. No. 285,103, filed Dec. 15, 1988. These applications disclose the preparation, characterization and utility of copper aluminum borate and are hereby incorporated by reference.
By way of general background, McArthur, in U.S. Pat. Nos. 3,856,702, 3,856,705 and 4,024,171, discloses that it has long been the practice in the art to impregnate or otherwise distribute active catalytic metals upon support materials having desired properties of porosity, surface area, thermal and mechanical stability, and suitably inert chemical properties. McArthur teaches that a superior catalyst support results from calcining certain aluminaboria composites within the temperature range of about 1,250.degree. C.-2,600.degree. F., which appears to produce a definite crystalline phase of 9 Al.sub.2 O.sub.3.2B.sub.2 O.sub.3 and also, in most cases, a crystalline phase of 2 Al.sub.2 O.sub.3.B.sub.2 O.sub.3, following which the aluminum borate support can be impregnated with solution(s) of desired catalytic salt or salts, preferably those that are thermally decomposable to give the corresponding metal oxides. Following impregnation, the finished catalysts are dried and, if desired, calcined at temperatures of, e.g., 500.degree. to 1000.degree. F. In the final catalyst, the active metal or metals may appear in the free form as oxides or sulfides or any other active form. Examples 1 to 6 of McArthur impregnate the calcined support with an aqueous solution of copper nitrate and cobalt nitrate to provide about 4% copper as CuO and 12% cobalt as Co.sub.2 O.sub.3 in the final catalyst.
Addressing preparation of the aluminum-boria support, McArthur states that conventional compounding procedures may be employed in compositing the alumina and the boria. He explains that it is necessary to provide an intimate admixture of the finely divided materials such as may be achieved by grinding, mulling, or ball milling the dry powders together, following which the mixture is shaped into a porous, self-supporting aggregate, as by tableting, prilling, extruding, casting or other well-known techniques to form cylindrical pellets or extrudates, spheres or other granular forms. Nothing in McArthur discloses or suggests a method for preparing copper aluminum borate by forming an aqueous composition comprising a volatile organic liquid, a source of copper(II) ions, a source of alumina, and a source of boria, drying the composition to form a copper aluminum borate precursor which when calcined at a sufficiently high temperature produces crystalline copper aluminum borate, such being distinguishable from McArthur's catalyst preparation which involves initial formation of the calcined alumina-boria support followed by post-treatment with the active metal.
Uhlig discloses preparation of a green tetragonal solid copper aluminum borate having the structure Cu.sub.2 Al.sub.6 B.sub.4 O.sub.17 in Diplomarbeit, Institute for Crystallography, Aacken (October 1976) "Phasen - und Mischkristall - Bildung im B.sub.2 O.sub.3 - armeren Teil des Systems Al.sub.2 O.sub.3 -CuO-B.sub.2 O.sub.3 " ("Formation of Phases and Mixed Crystals in that Part of the Al.sub.2 O.sub.3 -CuO-B.sub.2 O.sub.3 System With a Low B.sub.2 O.sub.3 Content") which is hereby incorporated by reference, by grinding together solid boron oxide, copper oxide and alumina, sealing the ground metal oxides in a platinum tube and heating same at 1000.degree. C. over the heating period of 48 hours. Attempts to produce this copper aluminum borate by the indicated route yield well-defined, dense crystalline particles which have an extremely low surface area and are accordingly not suitable for many catalysis processes due to the low porosity and surface area.
Asano, in U.S. Pat. No. 3,971,735, discloses a copper, zinc, aluminum and boron catalyst useful in low temperature methanol synthesis. The catalyst is preferably produced by forming a mixture of water-soluble salts of copper, zinc and boron, precipitating same with an alkali carbonate and mixing with alumina. The catalyst is then fired at approximately 300.degree.-450.degree. C.
Commonly assigned Satek U.S. Pat. No. 4,590,324 discloses preparation of catalytically active copper aluminum borate in a liquid medium which comprises (1) combining a source of divalent copper, trivalent aluminum and boron in the form of the oxide or borate, (2) drying the composition to remove water or diluent if necessary and (3) calcining the composition at a temperature sufficiently high to form crystalline copper aluminum borate having an X-ray diffraction pattern of Cu.sub.2 Al.sub.6 B.sub.4 O.sub.17. Satek states that, while copper aluminum borate can be prepared by various techniques, it is generally preferred to combine the oxide precursor reagents in an aqueous medium and that the presence of volatile components in the preparation of copper aluminum borate, such as water, NH.sub.3, acetate, etc., is advantageous in providing the copper aluminum borate with sufficient surface area and porosity for catalysis.
Recently, commonly assigned De Simone et al. U.S Pat. No. 4,755,497, disclosed a solid-state (i.e., dry) preparation of copper aluminum borate in which a superficially dry mixture comprising alumina, boria, and the desired metal oxide are calcined to form crystalline copper aluminum borate. Further, De Simone et al. disclose that the solid reagents comprising precursors of copper aluminum borate should be ground to a powder, individually or as a combination, through a 0.25 mm screen in a high speed grinder and it is important that uniform particle sizes of all reagents be attained in order that the solid-state reaction to form crystalline copper aluminum borate proceeds as uniformly as possible upon calcination. In addition to problems in attaining uniform particle sizes of the several reagents, recent work has determined there is a number of blending and processing problems confronting preparation of catalyst by the solid-state process. First, blending of the oxide precursors is difficult to control. Also there is difficulty in achieving homogeneity of dry-mixed solid reagents which is important for reproducibility of catalytic properties. A further problem is in obtaining catalysts having high surface area at the calcination temperatures required to form crystalline copper aluminum borate from dry-mixed solid reagents.
Accordingly, there is a need for a reproducible aqueous-organic process for production of copper aluminum borate capable of producing high surface area catalysts. Before the present invention, the aqueous preparation of copper aluminum borate, generally described in Satek, was complicated by sensitivity of the thixotropic aqueous mixture or gel to agitation and a tendency of the mixture to undergo phase separation upon drying. Stability in the gel and homogeneity in the dry solids are believed to be important for catalyst reproducibility.
An additional need exists for a convenient method of preparing copper aluminum borate which results in a catalyst for chemical conversion of organic compounds, e.g., o-ethylaniline to indole, o-ethylphenol to benzofuran, p-cymene to p-methyl-alpha-methylstyrene, methane to chloromethanes, p-ethyltoluene to p-methylstyrene, cumene to p-methylstyrene, alpha-ethyl-naphthalene to acenaphthene, etc., which is susceptible to enhancement by doping with active metals.
Satek discloses that the optimum copper aluminum borate catalyst for dehydrogenating alkylaromatics will vary for each individual feed. Consistent with this, our recent work with the previously disclosed solid-state and aqueous prepared catalyst has encountered the problem that, where the catalyst is intended for use in dehydrogenation of p-cymene to p-methyl-alpha-methylstyrene and/or oxychlorinaton of methane with hydrochloric acid and oxygen, incorporation into the catalyst of active metals as a means of improving catalyst performance is accompanied by the unwanted side effect of lowering selectivity of the catalyst.
It is therefore a general object of the present invention to provide an improved method for preparing copper aluminum borate and, in particular, to provide an economical and reproducible aqueous-organic method for preparing a dry-solid precursor of crystalline copper aluminum borate such that the resulting catalyst is at least comparable to that produced according to techniques previously disclosed.
It is a further object of the invention to provide an improved method for preparing copper aluminum borate which results in a catalyst for conversion of methane to chloromethanes which can be markedly enhanced by incorporation of relatively small amounts of active metals. Other objects appear hereinafter.
Throughout the present specification and claims, the terms "miscibility of organic liquids with water," etc., are intended to denote a classification of organic liquids such that when about 5 mL of an organic liquid and about 5 mL of water are shaken together in a test tube for 1 minute, and then the mixture allowed to settle, no liquid-liquid interfacial meniscus is observed. The terms "partial miscibility of organic liquids with water," etc., are intended to denote a classification of organic liquids such that when about 5 mL of an organic liquid and about 5 mL of water are shaken together in a test tube for 1 minute, and then the mixture allowed to settle, a liquid-liquid interfacial meniscus is observed and the volume of the aqueous phase is larger than the volume of the organic phase. The terms "precursor," "copper aluminum borate precursor," "dry-solid precursor," etc., denote compositions which, upon calcination at a sufficiently high temperature, result in crystalline copper aluminum borate. The terms "chemical base," "base," etc., are intended to denote any substance which will accept a proton in the relevant aqueous/organic solvent.
In the discussion that follows, reference is made to Temperature Programmed Reduction. As discussed in Zletz copending U.S. Pat. No. 4,729,979 and Satek U.S. Pat. No. 4,590,324 (hereby incorporated by reference), this test was carried out by placing 1.5.times.10.sup.-4 moles of copper aluminum borate in a 0.6 mm outside diameter vycor tube heated by an electric furnace. The tube was purged with helium or argon by heating to 300.degree. C. After cooling to ambient temperature, the gas feed to the vycor tube was switched to either 5% carbon monoxide in helium or 5% hydrogen in argon and the temperature was ramped to about 850.degree. C. at 8.degree. C./min. The temperature was controlled and ramped by a programmer equipped with a temperature controller. The change in gas composition of the effluent was detected with a thermal conductivity cell equipped with output to a strip-chart recorder. The carbon dioxide formed was removed from the effluent by a bed of ascarite and the water formed was removed by magnesium perchlorate. Unless otherwise stated, pore volume, surface area and average pore radius was determined by BET nitrogen adsorption (desorption test).